1. Field of the Invention
This invention relates to pressure-sensitive adhesives and adhesive tapes, particularly acrylic pressure-sensitive adhesives and adhesive tapes cured by ultraviolet radiation.
2. Description of the Related Art
The acrylate copolymer pressure-sensitive adhesives, with which the present invention is concerned, are well-known in the art (see for example in U.S. Pat. No. Re. 24,906 Ulrich). They are generally copolymers of a major proportion of alkyl esters of acrylic acid (the alkyl group containing from about four to fourteen carbon atoms) and a minor proportion of at least one modifying monomer such as acrylic acid, methacrylic acid, acrylamide, acrylonitrile, methacrylonitrile, N-substituted acrylamides, hydroxy acrylates, N-vinyl pyrrolidone, maleic anhydride or itaconic acid. They are among the most widely utilized adhesives in the manufacture of pressure-sensitive tapes for a variety of reasons including the ready availability and relatively low cost of the monomeric precursors which react easily to form copolymers that possess a good balance of tack, peel, and shear properties.
U.S. Pat. No. 4,181,752 (Martens et al.) discloses a process for making pressure-sensitive adhesive tape which involves the photopolymerization of the alkyl esters of acrylic acid and the modifying monomers to form the acrylate copolymer. Martens et al. disclose that the intensity and spectral distribution of the irradiation must be controlled in order to attain desirably high cohesive strength and also to attain high peel resistance. It teaches that the polymerizable mixture should be subjected to radiation in the near ultraviolet region at a rate of irradiation in the 300-400 nanometer wavelength range of not more than 7 milliwatts per square centimeter of the mass exposed. Any radiation shorter than 300 nanometers is limited to not more than about 10% of the energy in the 300-400 nanometers. The irradiation is preferably carried out in the absence of air and oxygen which inhibit the polymerization reaction. Thus, it is normally carried out in an inert atmosphere such as nitrogen, carbon dioxide, helium, argon, etc. Air can also be excluded by sandwiching the liquid polymerizable mixture between layers of solid sheet material and irradiating through the sheet material.
Additional patents further disclose ultraviolet radiation polymerization of acrylate adhesives using the process of Martens et al. U.S. Pat. No. 4,303,485 (Levens) discloses the addition of an oxidizable tin salt to the polymerizable mixture which is to be subjected to ultraviolet radiation polymerization to permit polymerization of thick layers in the presence of oxygen and to allow an unusual tolerance of oxygen when polymerizing thin layers. U.S. Pat. No. 4,364,972 (Moon) discloses the use of 15 to 50 parts by weight N-vinyl pyrrolidone as the modifying monomer in the ultraviolet radiation polymerizable mixture to provide a pressure-sensitive adhesive tape which has both high adhesion and high cohesion values and adheres strongly to automotive paints and to rubber and plastic foam layers. U.S. Pat. No. 4,391,687 (Vesley) discloses the use of specified chromophore-substituted-halomethyl-s-triazines as photoactive crosslinking agents in the ultraviolet radiation polymerizable monomer mixture with these triazines having good solubility in the monomer mixture and reduced tendency to yellowing and providing improved tolerance to oxygen during polymerization. U.S. Pat. No. 4,599,265 (Esmay) discloses a readily peelable pressure-sensitive adhesive tape, the adhesive layer of which is an ultraviolet radiation polymerized alkyl acrylate polymer which is crosslinked and nearly free from polar substituents. These patents also suggest that such conventional additives as tackifiers may be included in the adhesive, but do not exemplify this teaching.
The above-cited Moon patent, which concerns pressure-sensitive adhesive designed especially to provide enhanced adhesion to automotive paints, teaches that tackifiers can be blended into the photoactive mixtures of monomers from which those pressure-sensitive adhesives are photopolymerized, but warns that "the addition of any such material adds complexity and hence expense to an otherwise simple, straight forward, economical process and is not preferred except to achieve specific results" (col. 6, lines 3-12). The Moon patent does not exemplify this teaching. However, the introduction of a tackifier into a photopolymerizable mixture of monomers often interferes with the polymerization and prevents the attainment of the desired adhesive and cohesive properties.
U.S. Pat. No. 4,243,500 (Glennon) discloses a pressure-sensitive adhesive formed from a composition comprising mono-functional unsaturated acrylate ester monomer, essentially saturated tackifying resin polymer dissolved in the acrylate ester, non-crystallizing elastomeric material also dissolved in the acrylate ester, and an initiator responsive to ultraviolet light or other penetrating radiation such as electron beam, gamma, or X-ray radiation. Glennon discloses use of ultraviolet light within a wavelength range of between about 1800 and 4000 Angstroms and desirably between about 3500 and 3600 Angstroms. The adhesive composition is coated on a substrate and exposed to 200 watt per inch ultraviolet lamps. The intensity of these 200 watt per inch lamps taught by Glennon is much greater than the lamps disclosed by Martens et al. which provide an intensity of about 1 watt per lineal inch. Glennon discloses that the essentially saturated tackifying resin polymer can be a substance or mixture of substances selected from the group consisting of esters of rosin, hydrogenated esters of rosin, modified rosin esters, esters of polymerized rosin, esters of hydrogenated rosin, hydrocarbon resin, linear homo polymers of alpha-methyl styrene, alpha-pinene terpene hydrocarbon resin, aromatic modified C-5 hydrocarbon resin, vinyltoluene alpha methyl styrene copolymer resins, beta-pinene terpene resins, polycyclic hydrocarbon resins and technical hydroabietyl alcohol. However, many of these essentially saturated resin polymers are unsuitable for use in the curing method of the above-cited Martens patent due to incompatibility, which results in phase separation of the tackifying resin from the monomer mixture, excessive UV absorption which retards the photochemical reaction, and high reactivity with the monomers such that polymerization of the monomers is impeded.
U.S. Pat. No. 4,500,683 (Hori et al.) discloses a pressure-sensitive adhesive composition containing as a polymer component, an addition-polymerization polymer of an acryl-based polymer having sticking properties at room temperature and one or more ethylenically unsaturated monomers capable of forming a homo- or copolymer having a glass transition point of at least 273.degree. K. The addition-polymerization polymer is prepared by polymerizing one or more ethylenically unsaturated monomers in the presence of the acryl-based polymer by solution polymerization or bulk polymerization using radical polymerization catalysts, but polymerization can be initiated by energy in the form of light, electron rays, etc. Compounding agents such as a coloring agent, a filler, an anti-aging agent, a tackifier, etc. can be added.
U.S. Pat. No. 4,418,120 (Kealy et al.) discloses a pressure-sensitive adhesive tape which is made by coating a sheet backing with a solution of isooctyl acrylate:acrylic acid copolymer containing a tackifying rosin ester and an antioxidant, evaporating the solvent, and crosslinking the adhesive. U.S. Pat. No. 4,645,711 (Winslow et al.) discloses a removable pressure-sensitive adhesive tape, the adhesive layer of which is an emulsion polymerized copolymer of alkyl acrylate such as isooctyl acrylate and a small amount of emulsifier monomer and a tackifying resin selected from hydrogenated rosin esters, polyterpene, polymerized alkyl styrene, and polymerized petroleum-derived monomer resins. Although tackifying resins, such as the rosin esters have been successfully used with solution and emulsion polymerized acrylate pressure-sensitive adhesives and polyterpene, polymerized alkyl styrene, and polymerized petroleum-derived monomer resins can be used with emulsion polymerized acrylate pressure-sensitive adhesives, most of these tackifying resins are unsuitable for use in in situ polymerized acrylate pressure-sensitive adhesives.
Although acrylate adhesives generally have a good balance of tack, peel, and shear properties, an increase in these properties is desirable for the more demanding applications such as, for example, those applications requiring adhesion to low energy substrates such as polyethylene and polypropylene, and high solids automotive paint systems which are coming into widespread use to reduce air pollution. The tack property relates to the adhesive's ability to adhere quickly, the peel property relates to the adhesive's ability to resist removal by peeling, and the shear property relates to the adhesive's ability to hold in position when shear forces are exerted. Generally, the tack and peel properties are directly related to each other but are inversely related to the shear property. Typically, tackifying agents yield a 30% increase in adhesion, however, if an adhesive is modified to increase tack, its resistance to shear is lowered, and commonly an increase in shear resistance is accompanied by a reduction in tack. (See U.S. Pat. No. 4,077,926, Sanderson et al.)